The human body has two main biochemical pathways of "burning" fuel to supply energy for the performance of work. One of these pathways, referred to as aerobic metabolism, utilizes oxygen to produce energy whereas anaerobic metabolism does not utilize oxygen. Aerobic exercise, which requires the transport of large amounts of oxygen is believed to have beneficial effects on the cardiovascular system and is sometimes also referred to as cardiovascular exercise. Activities such as jogging, cycling, swimming, rowing and cross country skiing are generally identified as aerobic exercises and primarily involve the use of the muscles of the extremities, that is the arms and legs. Anaerobic exercise is sometimes referred to as resistance exercise and includes weight lifting and sprinting. Aerobic exercise is usually of moderate intensity, perhaps 60-80% of maximum exertion, is generally believed to require an extended duration of at least 15 or 20 minutes and burns primarily fat (fatty acids) for fuel. Aerobic exercise is believed to improve endurance, decrease blood pressure and body mass and improve the lipid profile and insulin sensitivity. On the other hand, anaerobic exercise requires maximum intensity usually in the 80-100% of the possible intensity range, is usually performed in seconds to minutes, and utilizes primarily sugar and glycogen stores for fuel. Anaerobic exercise tends to increase both body strength and size.
Known forms of aerobic exercise machines include treadmills, exercise cycles, stair steppers, rowers, cross country skiers and arm ergometer (arm cycles) which exercise primarily the muscles of the legs and arms. Known weight training machines for anaerobic exercise use weights, elastic bands, springs and pressurized canisters which in most cases exercise the extremities. However, a few resistance or anaerobic machines do provide unidirectional resistance training for either the abdominal or back muscles, such as the abdominal machines and back extension machines manufactured and marketed by Nautilus, Cybex, Universal and Life Fitness as well as some other exercise equipment companies. Several flexor or extensor chairs or machines have also been patented including those shown and described in U.S. Pat. Nos. 3,767,190-Biggerstaff; 5,110,121-Foster and 5,215,511-Cheng. However, none of the prior art has taught nor suggested an aerobic exercise machine which provides a substantial uniform resistance on both the flexion and extension portions of the cycle utilizing an aerobic level of resistance.